In humans with advance cancer, anti-tumour immunity is often ineffective due to the tightly regulated interplay of pro- and anti-inflammatory, immune-stimulatory and immunosuppressive signals. For example, loss of the anti-inflammatory signals leads to chronic inflammation and prolonged proliferative signalling. Interestingly, cytokines that both promote and suppress proliferation of the tumour cells are produced at the tumour site. It is the imbalance between the effects of these various processes that results in tumour promotion.
To date, a major barrier to attempts to develop effective immunotherapy for cancer has been an inability to break immunosuppression at the cancer site and restore normal networks of immune reactivity. The physiological approach of immunotherapy is to normalize the immune reactivity so that, for example, the endogenous tumour antigens would be recognized and effective cytolytic responses would be developed against tumour cells. Although it was once unclear if tumour immunosurveillance existed, it is now believed that the immune system constantly monitors and eliminates newly transformed cells. Accordingly, cancer cells may alter their phenotype in response to immune pressure in order to escape attack (immunoediting) and upregulate expression of inhibitory signals. Through immunoediting and other subversive processes, primary tumour and metastasis maintain their own survival.
One of the major mechanisms of anti-tumour immunity subversion is known as ‘T-cell exhaustion’, which results from chronic exposure to antigens and is characterized by the up-regulation of inhibitory receptors. These inhibitory receptors serve as immune checkpoints in order to prevent uncontrolled immune reactions.
PD-1 and co-inhibitory receptors such as cytotoxic T-lymphocyte antigen 4 (CTLA-4, B and T Lymphocyte Attenuator (BTLA; CD272), T cell Immunoglobulin and Mucin domain-3 (Tim-3), Lymphocyte Activation Gene-3 (Lag-3; CD223), and others are often referred to as a checkpoint regulators. They act as molecular “tollbooths,” which allow extracellular information to dictate whether cell cycle progression and other intracellular signalling processes should proceed.
In addition to specific antigen recognition through the TCR, T-cell activation is regulated through a balance of positive and negative signals provided by co-stimulatory receptors. These surface proteins are typically members of either the TNF receptor or B7 superfamilies. Agonistic antibodies directed against activating co-stimulatory molecules and blocking antibodies against negative co-stimulatory molecules may enhance T-cell stimulation to promote tumour destruction.
Programmed Cell Death Protein 1, (PD-1 or CD279), a 55-kD type 1 transmembrane protein, is a member of the CD28 family of T cell co-stimulatory receptors that include immunoglobulin superfamily member CD28, CTLA-4, inducible co-stimulator (ICOS), and BTLA. PD-1 is highly expressed on activated T cells and B cells. PD-1 expression can also be detected on memory T-cell subsets with variable levels of expression. Two ligands specific for PD-1 have been identified: programmed death-ligand 1 (PD-L1, also known as B7-H1 or CD274) and PD-L2 (also known as B7-DC or CD273). PD-L1 and PD-L2 have been shown to down-regulate T cell activation upon binding to PD-1 in both mouse and human systems (Okazaki et al., Int Immunol., 2007; 19: 813-824). The interaction of PD-1 with its ligands, PD-L1 and PD-L2, which are expressed on antigen-presenting cells (APCs) and dendritic cells (DCs), transmits negative regulatory stimuli to down-modulate the activated T cell immune response. Blockade of PD-1 suppresses this negative signal and amplifies T cell responses.
Numerous studies indicate that the cancer microenvironment manipulates the PD-L1-/PD-1 signalling pathway and that induction of PD-L1 expression is associated with inhibition of immune responses against cancer, thus permitting cancer progression and metastasis. The PD-L1/PD-1 signalling pathway is a primary mechanism of cancer immune evasion for several reasons. First, and most importantly, this pathway is involved in negative regulation of immune responses of activated T effector cells, found in the periphery. Second, PD-L1 is up-regulated in cancer microenvironments, while PD-1 is also up-regulated on activated tumour infiltrating T cells, thus possibly potentiating a vicious cycle of inhibition. Third, this pathway is intricately involved in both innate and adaptive immune regulation through bi-directional signalling. These factors make the PD-1/PD-L1 complex a central point through which cancer can manipulate immune responses and promote its own progression.
The first immune-checkpoint inhibitor to be tested in a clinical trial was ipilimumab (Yervoy, Bristol-Myers Squibb), an CTLA-4 mAb. CTLA-4 belongs to the immunoglobulin superfamily of receptors, which also includes PD-1, BTLA, TIM-3, and V-domain immunoglobulin suppressor of T cell activation (VISTA). Anti-CTLA-4 mAb is a powerful checkpoint inhibitor which removes “the break” from both naïve and antigen-experienced cells. Therapy enhances the antitumor function of CD8+ T cells, increases the ratio of CD8+ T cells to Foxp3+ T regulatory cells, and inhibits the suppressive function of T regulatory cells. The major drawback to anti-CTLA-4 mAb therapy is the generation of autoimmune toxicities due to on-target effects of an over-exuberant immune system which has lost the ability to turn itself down. It has been reported that up to 25% of patients treated with ipilimumab developed serious grade 3-4 adverse events/autoimmune-type side effects including dermatitis, enterocolitis, hepatitis, endocrinopathies (including hypophysitis, thyroiditis, and adrenalitis), arthritis, uveitis, nephritis, and aseptic meningitis. In contrast to the anti-CTLA-4 experience, anti-PD-1 therapy appears to be better-tolerated and induces a relatively lower rate of autoimmune-type side effects.
TIM-3 has been identified as another important inhibitory receptor expressed by exhausted CD8+ T cells. In mouse models of cancer, it has been shown that the most dysfunctional tumour-infiltrating CD8+ T cells actually co-express PD-1 and TIM-3.
LAG-3 is another recently identified inhibitory receptor that acts to limit effector T-cell function and augment the suppressive activity of T regulatory cells. It has recently been revealed that PD-1 and LAG-3 are extensively co-expressed by tumour-infiltrating T cells in mice, and that combined blockade of PD-1 and LAG-3 provokes potent synergistic antitumor immune responses in mouse models of cancer.
PD-1 pathway blockade can be combined with vaccines or other immunomodulatory antibodies for improved therapeutic efficacy (Hirano, F. et al, Cancer Res., 65(3): 1089-1096 (2005); Li, B. et al, Clin. Cancer Res., 15: 1507-1509 (2009); and Curran, M. A. et al, Proc. Natl. Acad. Set, 107(9):4275-4280 (2010)).
Currently, antagonist mAbs against both PD-1 and their ligand PD-L1 are in various stages of development for the treatment of cancer, and recent human trials have shown promising results in cancer patients with advanced, treatment-refractory disease.
The first of the agents blocking the B7-H1/PD-1 pathway to enter phase I clinical trials was Nivolumab (MDX-1106/BMS-936558/ONO-4538), a fully human IgG4 anti-PD-1 mAb developed by Bristol-Myers Squibb. Another PD-1 mAb undergoing clinical evaluation is CT-011, a humanized IgG1 mAb specific for PD-1 developed by CureTech Ltd. Other agents include Lambrolizumab (MK-3475—Merck), a humanized monoclonal IgG4 PD-1 antibody; BMS-936559, a fully human IgG4 PD-L1 antibody and Roche's MPDL3280A, a human monoclonal antibody that targets the PD-L1 pathway.
Accordingly, an aim of the present invention is a combination therapy for treating cancer comprising an immunomodulator and blockade of checkpoint inhibitors with the potential to elicit potent and durable immune responses.